Z-type films for second harmonic generation

ABSTRACT

Optically non-linear materials suitable upon repeated Langmuir-Blodgett dipping for forming Z-type (head-to-tail) films for second harmonic generation include hemicyanine dyes which are very unsymmetrical, i.e. which have hydrophobic substituents differing in size by at least 4 carbon atoms, each end of a hydrophilic chromophore. A suitable dye is E-N-octadecyl-4- 2-(4-dibutylaminophenyl)ethenyl!quinolinium iodide.

The present invention relates to optically non-linear materials suitablefor forming Z-type films for second harmonic generation, to methods fortheir synthesis and to optical devices which incorporate such materials.

WO 89/05843 discloses amphiphilic hemicyanines such as 4-{2-4'-(N,N-didecylamino)-biphenyl-4,4'-diyl!-ethenyl}-1-butylpyridiniumiodide, useful as non-linear optical media, which may formLangmuir-Blodgett films of non-centrosymmetric chromophores.

EP-A-203780 discloses a multi-layer Langmuir-Blodgett film havingnon-linear optical properties, consisting of alternating layers ofdifferently substituted asymmetric hemicyanine dyes.

EP-A423619 discloses various compounds which can be deposited byLangmuir-Blodgett techniques to form films for non-linear-opticspurposes.

Film-forming hemicyanine dyes of general formula D--C₆ H₄ --CH═CH--C₅ H₄N⁺ --R X⁻ where D is an electron donating group, R is a hydrophobicalkyl group and X⁻ is a simple inorganic anion such as a halide ion havebeen extensively studied for second harmonic generation (SHG). Theirproperties are dependent upon the donor group (Bubeck et al, AdvancedMaterials 1991, 3, 54), the counterion (Liu et al, Thin Solid Films1992, 219 221; Ashwell et al Nature 1992, 357 393) and the extent ofaggregation (Girling et al, Thin Solid Films 1985, 132, 101; Schildkrautet al., Opt. Lett 1988, 13 134). In our International PCT Publication WO93/24859 we have disclosed that Langmuir-Blodgett (LB) film structureswhich generate a signal of greater intensity may be produced byutilising an amphiphilic anion rather than a simple inorganic anion. Forthese known LB film-forming materials, it has been assumed that there isa requirement for the cation to be amphiphilic with a long hydrophobictail group attached to one end of the hydrophilic chromophore, thisbeing necessary so that the film-forming molecules may be aligned at theair/water interface, with the head groups downwards. Such filmstypically display centrosymmetric interlayer arrangements with themolecules packing head-to-head and tail-to-tail (Y-type). The moleculestend to "flip" over to retain the hydrophobic parts together but anon-centrosymmetric structure has been obtained by the incorporation ofspacer layers between the active LB layers (International PatentApplication WO 92/09374).

We have now discovered that, in film forming hemicyanine dyes, a morefavourable non-centrosymmetric, head-to-tail, Z-type, packingarrangement may be produced by ensuring that the hydrophilic chromophoreis substituted at opposite ends by hydrophobic alkyl, alkenyl oralkylaryl groups comprising from 4 (less than 4 would be insufficientlyhydrophobic) to 30 carbon atoms, in which the groups at each end differin size by four or more carbon atoms, e.g. dihexyl (12C) and octadecyl(18C), difference=6C. A difference of at least four carbon atoms isnecessary for Z-type packing, if the inconvenience of intermediatelayers of a different compound is to be avoided.

Preferred cationic dyes of use in the present invention are those ofgeneral formula R'D--π--A--R (neutral) or R'D--π--A⁺ R (cationic)wherein R'D represents a hydrophobic donor group; π represents aconjugated π-electron bridging group; A and A⁺ represent electronacceptor groups having hydrophobic substituents R.

Preferably R'D represents an N-alkylamino, N,N-diallylamino, alkoxy,carboxylate R'C(O)O--, alkylthio or alkylseleno group and A-R representsa keto (RC(O)--) or ester (ROC(O)--) group. A⁺ represents a heterocyclicgroup such as a pyridinium, pyrimidinium, quinolinium, isoquinolinium,benzothiazolium or thiazolium cation and R is preferably attached to theheteroatom. A⁺ may be optionally substituted.

R and R' represent hydrophobic alky, alkenyl or alkylaryl groupscomprising 4 to 30 carbon atoms, R and R' being the same or different,optionally differing by four or more carbon atoms. The π-bridge is--CH═CH--, --CH═N--, --N═N--, --Ar-- or multiples or combinationsthereof, wherein Ar represents an aromatic ring especially a benzene orpyridine ring, the π group being optionally substituted.

The counterion may be any conventional anion of use in suchLangmuir-Blodgett films. Preferably, the anion is a halide or anamphiphilic anion such as those disclosed in UK Patent ApplicationPublication GB 2267288A.

Examples of dyes of the type described include

E-N-octadecyl-4- 2-(4-dibutylaminophenyl)ethenyl!quinolinium iodide,

E-N-octadecyl-4- 2-(4-dibutylaminophenyl)ethenyl!pyridinium iodide,

E-N-octadecyl-4- 2-(4-dihexylaminophenyl)ethenyl!pyridinium iodide,

E-N-decyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide,

E-N-dodecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide,

E-N-tetradecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide,

E-N-hexadecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide,

E-N-octadecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide,

E-N-eicosyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide,

E-N-docosyl-4- 2-(4-tridecanoyloxyphenyl)ethenyl!pyridinium bromide,

E-N-dodecyl-4- 2-(4-docosylthiophenyl)ethenyl!pyridinium iodide,

E-N-dodecyl-2- 2-(4-octadecylaminophenyl)ethenyl!benzothiazolium iodide,

E-N-hexadecyl-2- 2-(4-docosylaminophenyl)ethenyl!benzothiazoliumoctadecylsulphate, and

E-N-octadecyl-5-(4-dibutylaminophenyl)methylene!-6,7,8-trihydroisoquinoliniu moctadecylsulphate.

The novel dyes of this invention (as listed above) may be convenientlyprepared by, for example, the reaction of an N-alkyl-4methylpyridiniumhalide, a para-substituted benzaldehyde and piperidine in hot methanol.The product may be purified or may be used directly in the production ofoptically non-linear Langmuir-Blodgett films. Alternatively the dyes canbe mixed with, for example, sodium octadecylsulphate on the aqueoussubphase of the Langmuir-Blodgett trough to form the octadecylsulphatesalt. The inorganic salt produced by the metathesis reaction migratesinto the subphase.

The novel dyes may be formed into optically non-linear films usingconventional techniques and a further aspect of the invention relates tosuch films. Conveniently they may be spread as a solution in dilutemethanol/chloroform or dichloromethane onto the pure water subphase of aNima Technology Trough and transferred on the upstroke to a glass side.

The invention is illustrated by the following examples.

EXAMPLE 1

The optically non-linear dye, E-N-octadecyl-4-2-(4-butylaminophenyl)ethenyl!-quinolinium iodide, was spread fromAristar grade dichloromethane onto the pure water subphase ofcompartment A of a Nima Technology LB trough. After evaporation of thesolvent the surface layer was slowly compressed to 35 mN/m, and then itwas transferred to a glass microscope slide by raising it at 3 mm/minthrough the floating monolayer. Z-type film structures were obtained byrepeating the process, for example, by cycling the slide via compartmentB, under the fixed surface barrier and up through the floatingmonolayer.

SHG measurements were carried out using the apparatus describedpreviously (Ashwell et al., SPIE--Intl Soc. Opt. Eng. 1991, 1361, 569),with a pulsed Nd:YAG laser (λ 1064 nm) and Y-cut quartz plate asreference. The SHG increased quadratically with the number of layersindicating a non-centrosymmetric LB film structure.

EXAMPLE 2

Non-centrosymmetric Z-type multilayers of the related pyridinium dye,E-N-octadecyl4 -2-(4-dibutylaminophenyl)ethenyl!pyridinium iodide, wereobtained using the technique described above. The SHG increasedquadratically with the number of layers.

EXAMPLE 3

Non-centrosymmetric Z-type multilayers of E-N-octadecyl-4-2-(4-dihexylaminophenyl)ethenyl!pyridinium iodide, were obtained usingthe technique described above. The SHG increased quadratically with thenumber of layers.

EXAMPLE 4

Non-centrosymmetric Z-type multilayers of E-N-alkyl-4-2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide, wherein the alkylgroup was, in successive experiments, decyl, dodecyl, tetradecyl,hexadecyl, octadecyl, eicosyl and docosyl, were obtained using thetechnique described above. In each experiment the SHG increasedquadratically with the number of layers.

EXAMPLE 5

Non-centrosymmetric Z-type multilayers of E-N-octadecy-4-2-(4-octadecyloxyphenyl)ethenyl!pyridinium iodide, were obtained usingthe technique described above. Films comprising from 1 to 120non-centrosymmetic layers were fabricated and the SHG increasedquadratically with the number.

I claim:
 1. A film forming hemicyanine dye comprising a hydrophilicchromophore substituted at both ends by hydrophobic alkyl, alkenyl oralkylaryl groups comprising from 4 to 30 carbon atoms, the dye havingthe formula: ##STR1## where (R¹)₂ D is a hydrophobic electron donatinggroup and A is an electron accepting group substituted by a hydrophobicgroup R, π is a conjugated bridging group and X⁻ is a counterion, inwhich R¹ is butyl to hexyl and R has at least sixteen carbon atoms.
 2. Adye according to claim 1, wherein A⁺ represents a heterocyclic group. 3.A dye according to claim 2, wherein A⁺ represents a pyridinium,pyrimidinium, quinolinium, isoquinolinium, oxazolium, thiazolium orbenzothiazolium cation.
 4. A dye according to claim 2, wherein R isattached to the heteroatom.
 5. A dye according to claim 4, wherein A⁺ issubstituted.
 6. A dye according to claim 1, wherein X⁻ is a halide.
 7. Adye according to claim 1, wherein X⁻ is amphiphilic.
 8. A dye accordingto claim 1, wherein AR represents a keto (RC(O)--) or an ester(ROC(O)--) group.
 9. A dye according to claim 1, wherein (R¹)₂ Drepresents a N,N-dialkylamino group.
 10. A dye according to claim 1,wherein π represents a group selected from --CH═CH--, --CH═N--, --N═N--,and Ar where Ar represents an aromatic ring or multiples or combinationsthereof.
 11. A dye according to claim 10 wherein π is substituted.
 12. Anon-centrosymmetric Langmuir-Blodgett film consisting of a plurality oflayers in Z-type arrangement of a dye being any one of the followingcompounds:E-N-octadecyl-4- 2-(4-dibutylaminophenyl)ethenyl!pyridiniumiodide; E-N-octadecyl-4- 2-(4-dibutylaminophenyl)ethenyl!quinoliniumiodide; E-N-octadecyl-4- 2-(4 dihexylaminophenyl)ethenyl!pyridiniumiodide; E-N-decyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromide;E-N-dodecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridium bromide;E-N-tetradecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromideE-N-hexadecyl-4- 2-(4-docosyloxphenyl)ethenyl!pyridinium bromideE-N-octadecyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromideE-N-eicosyl-4- 2-(4-docosyloxyphenyl)ethenyl!pyridinium bromideE-N-docosyl-4- 2-(4-tridecanoyloxyphenyl)ethenyl!pyridinium bromideE-N-dodecyl-4- 2-(4-docosylthiophenyl)ethenyl!pyridinium iodideE-N-dodecyl-2- 2-(4-octadecylaminophenyl)ethenyl!benzothiazolium iodide;E-N-hexadecyl-2- 2-(4-docosylaminophenyl)ethenyl!benzothiazoliumoctadecylsulphate; or E-N-octadecyl-5-(4-dibutylaminophenyl)methylene!-6,7,8-trihydroisoquinoliniumoctadecylsulphate.